CRC is like an error check on the integrity of a ROM. A clean dump will give you a certain CRC.
I believe you can find the CRC in the main mupen64 GUI (not the Picklelauncher) There is an option to select which columns to display in the rom lister, so enable the CRC column, and then point mupen to the rom directory and wait until it reads your roms. You should then be able to see each rom's CRC.